The present invention relates to apparatus having capacitive electrodes for measuring electrical characteristics in the subsoil.
DC electrical prospecting techniques have long shown their effectiveness in prospecting and surveying the subsoil. Those methods have been developed continuously for surveying from the surface of the ground. When implemented in boreholes, those techniques have become known as electrical logging, electrical tomography, electrical cylinder, . . . Whatever the particular technique that is in use, it is always based on using at least four electrodes: two xe2x80x9ccurrent injectionxe2x80x9d electrodes for injecting a current I into the ground, and two other electrodes (referred to as xe2x80x9cmeasurementxe2x80x9d or xe2x80x9cpotentialxe2x80x9d electrodes) between which the resulting potential difference is measured. It is the shape of the two-port network constituted by those four electrodes at their positions on the surface of the ground and/or in one or more boreholes that determine the particular name given to the technique.
In those electrical measurement techniques, it is desired to survey the subsoil over a thickness of several tens of meters at most; for that purpose, boreholes are drilled. The terrain is often heterogeneous, weathered, and the boreholes need to be cased. In addition, they are often above the level of the water table, at least in part, in which case they do not contain water. Furthermore, they do not always have a fluid that conducts electricity.
In other words, in such situations, it is not possible by conductive coupling to establish electrical circuit continuity between the various electrodes in use and the subsoil in which measurements are to be performed.
An object of the present invention is to provide apparatus for measuring the electrical characteristics of the subsoil from a borehole, which apparatus can be used in particular when no means are available for providing electrical continuity by conduction between the electrode(s) placed in the borehole(s) and the subsoil which is to be measured.
To achieve this object, the invention provides apparatus with capacitive electrodes for electrical prospection in a borehole, the apparatus comprising at least two injection electrodes and a current generator for injecting current into the ground via them, and also two measurement electrodes and means for measuring the potential difference between them, the apparatus being characterized in that:
said current generator delivers alternating current at a frequency preferably lying in the range 5 kHz to 100 kHz and at a voltage of less than 1000 volts;
at least one of the four electrodes is designed to be placed in a borehole and has an outer surface that is substantially cylindrical and electrically conductive;
xe2x80x83and in that it further comprises:
means for measuring the current flowing through said injection electrodes;
means for measuring the potential difference between said measurement electrodes; and
means for processing said current measurement and said potential difference measurement to deduce therefrom the characteristics of the subsoil in which the borehole is situated.
It will be understood that by means of the dispositions of the invention, resistive coupling is indeed established between the electrode(s) disposed in the borehole(s) and the subsoil in which measurement is to be performed, even though no means enable conduction to be provided by electrical continuity between the electrodes and the subsoil.
This coupling is of sufficiently good quality to enable the measurements taken to be usable for surveying the electrical characteristics of the subsoil that is to be prospected.
In a preferred embodiment, at least one of said injection electrodes is for placing in the borehole. It is then substantially cylindrical and has an electrically conductive outer surface.
In another preferred embodiment, the device for measuring the injected current and the injection electrode are mounted on a common component for placing in the borehole in the desired position(s), depending on the measurements that are to be performed.
It will be understood that by means of this disposition, the injected current can be measured with great accuracy, thereby facilitating interpretation of the results.
According to another preferred characteristic, at least one of said measurement electrodes is for placing in a borehole. It is then substantially cylindrical and has an electrically conductive outer surface, and it is secured to a portion of the device for measuring voltage.
This disposition makes it possible to improve the accuracy with which the potential difference between the measurement electrodes is measured.